Oxidized nanofibrillar cellulose is a cellulose derivative used as an additive in papermaking. The retention of the fibrils in the paper is difficult to determine due to lack of a reliable analytical method. Accordingly, there is a demand for an analytic method for the determination of the concentration of oxidized nanofibrillar cellulose and/or its carbonyl ratio in a material and thus the retention of oxidized nanofibrillar cellulose in the paper products.